Read the GNOME Human Interface Guidelines. While there are many details that differ between the two documents (and where there are differences this document, of course, takes precedence) the GNOME HIG contain a wealth of good advice on creating usable, attractive interfaces.

Application Installation and Location

File Attributes

General

Windows

Document Windows

Control Windows

Dialogue Windows

Menu Bar

Tool Bar

Status Bar

Context Menus

Text Messages

Shortcut Keys